Bonding agent, aluminum nitride composite body, and manufacturing method of the same

ABSTRACT

A bonding agent comprises a flux containing either calcium aluminate or calcium oxide and aluminum oxide and containing less than 5 wt % rare-earth elements; and aluminum nitride powder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. P2004-332377, filed on Nov. 16,2004, and No. P2005-234999, filed on Aug. 12, 2005; the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bonding agent to bond a plurality ofaluminum nitride sintered bodies, an aluminum nitride composite body,and a method of manufacturing the same.

2. Description of the Related Art

Conventional methods to bond aluminum nitride sintered bodies to eachother include a solid state bonding method and a solid-liquid statebonding method. In the solid state bonding method, a bonding agent isinterposed between the aluminum nitride sintered bodies, and bonding isperformed in a state where the bonding agent is not melted, that is, thebonding agent is solid. The solid state bonding can provide good bondingby heating at a high bonding temperature of 1850° C. or more (forexample, see Patent Literature 1).

In the solid-liquid state bonding method, a bonding agent is interposedbetween the aluminum nitride sintered bodies, and bonding is performedin a state where part of the bonding agent is melted and solid andliquid are mixed. Some types of the solid-liquid state bonding methodcan provide good bonding by using a flux containing 25 to 45 wt %calcium oxide, 5 to 30 wt % yttrium oxide, and balance aluminum oxideand heating the aluminum nitride sintered bodies at a bondingtemperature of 1650 to 1800° C. (for example, see Patent literatures 2and 3).

Moreover, a heater including a heating element and an electrostaticchuck including an electrode are manufactured using the above describedbonding methods. The heater and electrostatic chuck are used in acorrosive gas environment.

-   Patent Literature 1: Japanese Patent Laid-open Publication No.    8-73280-   Patent Literature 1: Japanese Patent Laid-open Publication No.    10-167850-   Patent Literature 1: Japanese Patent Laid-open Publication No.    10-273370

However, the solid state bonding method requires heating to a hightemperature of 1850° C. or more in order to obtain good bonding.Moreover, in the solid-liquid state bonding method using the fluxcontaining 25 to 45 wt % calcium oxide, 5 to 30 wt % yttrium oxide, andbalance aluminum oxide, the bonding temperature can be set lower thanthat of the solid state bonding. However, heating to 1650 to 1800 ° C.is required in order to obtain good bonding. As described above, thebonding temperature is high in either conventional bonding method,therefore the aluminum nitride sintered bodies might be deformed bybonding.

Moreover, when the bonding is performed in a manufacturing process ofthe heater or electrostatic chuck, the heating element or electrodesmight change in quality or the volume resistivity might change due tothe high bonding temperature. This might result in degradation ofproperties including temperature uniformity of the heater and uniformityof chucking force of the electrostatic chuck.

Furthermore, the high bonding temperature increases energy required forbonding or requires processing again after bonding due to deformation,thus increasing manufacturing costs.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a bonding agent foraluminum nitride sintered bodies which is capable of providing goodbonding at low bonding temperature, an aluminum nitride composite body,and a method of manufacturing the same.

A bonding agent according to the present invention includes a flux andaluminum nitride powder. The flux contains either calcium aluminate orcalcium oxide and aluminum oxide and contains less than 5 wt %rare-earth elements. Such a bonding agent can provide good bonding at alow bonding temperature of 1500° C. or less.

The calcium aluminate can contain, for example, at least one ofCa₁₂Al₁₄O₃₃ and Ca₃Al₂O₆.

Preferably, the flux contains 30 to 80 wt % calcium and 20 to 70 wt %aluminum. Preferably, the flux contains 0.01 to 5 wt % silica. Moreover,in the bonding agent, the content of the flux is 10 to 90 wt % and thecontent of the aluminum nitride powder is 10 to 90 wt %. Morepreferably, maximum particle diameters of the flux and the aluminumnitride powder are 45 μm or less.

An aluminum nitride composite body according to the present inventionincludes a plurality of aluminum nitride sintered bodies and a bondinglayer formed between each adjacent pair of the plurality of aluminumnitride sintered bodies. The bonding layer contains nitrogen, oxygen,aluminum and calcium and contains less than 15 wt % rare-earth elements.The plurality of aluminum nitride sintered bodies are bonded through thebonding layer. Such an aluminum nitride composite body can be obtainedat a low bonding temperature of 1500° C. or less, thus resulting insmall deformation of the aluminum nitride sintered bodies and goodbonding.

Preferably, the bonding layer contains 15 to 30 wt % nitrogen, 10 to 35wt % oxygen, 20 to 55 wt % aluminum, and 5 to 20 wt % calcium.

Moreover, the aluminum nitride composite body can be used in a heaterincluding a heating element or an electrostatic chuck including anelectrode. Such an aluminum nitride composite body can be obtained at alow bonding temperature of 1500° C. or less. Accordingly, the heatingelement and the electrode do not change in nature, and the volumeresistivity thereof scarcely changes at bonding. Such heater andelectrostatic chuck have therefore excellent capabilities.

A method of manufacturing an aluminum nitride composite body includes:heating a plurality of aluminum nitride sintered bodies with a bondingagent interposed therebetween at a bonding temperature of 1500° C. orless to melt the bonding agent; and bonding the plurality of aluminumnitride sintered bodies to each other. The bonding agent contains a fluxand aluminum nitride powder, and the flux contains either calciumaluminate or calcium oxide and aluminum oxide and contains less than 5wt % rare-earth elements. Such a manufacturing method can provide goodbonding at a low temperature of 1500° C. or less and reduce deformationof the aluminum nitride sintered bodies.

Preferably, the plurality of aluminum nitride sintered bodies with thebonding agent interposed therebeween are heated to the bondingtemperature at a heating rate of 0.5 to 10.0° C./min. Preferably, anaverage surface roughness of a bonding surface of each of the aluminumnitride sintered bodies is 0.1 to 2.0 μm. According to the presentinvention, it is possible to provide a bonding agent for the aluminumnitride sintered bodies which is capable of providing good bonding atlow bonding temperature, an aluminum nitride composite body, and amethod of manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an aluminum nitride compositebody according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a heater according to the embodimentof the present invention.

FIG. 3 is a cross-sectional view of an electrostatic chuck according tothe embodiment of the present invention.

FIG. 4 is an X-ray diffraction diagram of a flux according to theembodiment of the present invention.

FIG. 5 is a drawing-substitute photograph showing a result of SEMobservation of a composite body according to the embodiment of thepresent invention.

FIG. 6 is a drawing-substitute photograph showing a result of an EDSanalysis of a nitrogen distribution of the composite body according tothe embodiment of the present invention.

FIG. 7 is a drawing-substitute photograph showing a result of an EDSanalysis of an oxygen distribution of the composite body according tothe embodiment of the present invention.

FIG. 8 is a drawing-substitute photograph showing a result of an EDSanalysis of an aluminum distribution of the composite body according tothe embodiment of the present invention.

FIG. 9 is a drawing-substitute photograph showing a result of an EDSanalysis of a calcium distribution of the composite body according tothe embodiment of the present invention.

FIG. 10 is a drawing-substitute photograph showing a result of an EDSanalysis of an yttrium distribution of the composite body according tothe embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[Bonding Agent]

A bonding agent is used to bond a plurality of aluminum nitride sinteredbodies to each other. Bonding using the bonding agent of an embodimentis performed at a low bonding temperature of 1500° C. or less. Thebonding agent contains a flux and aluminum nitride (AlN) powder. Theflux is melted by heating at the bonding temperature.

The flux contains either calcium aluminate (Ca_(x)Al_(y)O_(z)) orcalcium oxide (CaO) and aluminum oxide (Al₂O₃). In other words, the fluxcontains at least calcium aluminate or contains both calcium oxide andaluminum oxide. Specifically, the flux contains only calcium aluminate;contains calcium aluminate and calcium oxide; contains calcium aluminateand aluminum oxide; contains calcium aluminate, calcium oxide, andaluminum oxide; or contains calcium oxide and aluminum oxide.

The flux can contain, as calcium aluminate (Ca_(x)Al_(y)O_(z)), at leastone of Ca₁₂Al₁₄O₃₃ (x=12, y=14, z=33) and Ca₃Al₂O₆ (x=3, y=2, z=6).Specifically, as calcium aluminate, the flux may contain onlyCa₁₂Al₁₄O₃₃, may contain only Ca₃Al₂O₆, or may contain both Ca₁₂Al₁₄O₃₃and Ca₃Al₂O₆. Moreover, in addition to Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆, theflux may contain calcium aluminate with a phase other than those ofCa₁₂Al₁₄O₃₃ and Ca₃Al₂O₆.

The content of rare-earth elements in the flux is set to less than 5 wt%. When the content of rare-earth elements, such as yttrium, in the fluxis 5 wt % or more, the aluminum nitride sintered bodies cannot be bondedat a bonding temperature of 1500° C. or less. Even if the aluminumnitride sintered bodies can be bonded, the strength at a bonded portionis low, and good bonding cannot be obtained. More preferably, the fluxdoes not contain rare-earth elements.

The flux can contain alkali metal oxides, alkaline-earth metal oxides,and oxides such as a grass forming material. Examples of the grassforming material which can be contained are silica (SiO₂), boron oxide(B₂O₃), phosphorous oxide (P₂O₃), and the like. For example, adding alittle amount of silica to the flux can further lower the bondingtemperature without reducing thermal cycle endurance and corrosionresistance. When the bonding agent is used to manufacture an aluminumnitride composite body for use in a corrosive gas environment, it ispreferable that the content of silica in the flux is 10 wt % or less.The content of silica of 10 wt % or less allows the corrosion resistanceto the corrosive gas to be maintained. More preferably, the content ofsilica is 0.01 to 5 wt %. This can lower the bonding temperature andincrease the strength of the bonded portion while maintaining theendurance and corrosion resistance.

The total content of oxides in the flux other than the oxide containingat least one of calcium and aluminum is, preferably, less than 20 wt %and, more preferably, 10 wt % or less. Still more preferably, the fluxdoes not contain oxides other than the oxide containing at least one ofcalcium and aluminum.

Preferably, the flux contains 30 to 80 wt % calcium and 20 to 70 wt %aluminum. Such contents of calcium and aluminum can increase thestrength of the bonded portion. More preferably, the flux contains 45 to70 wt % calcium and 30 to 55 wt % aluminum.

The purity of the aluminum nitride powder is preferably 95% or more and,more preferably, 99% or more. This can reduce the influence ofimpurities and can provide good bonding.

Preferably, the bonding agent contains 10 to 90 wt % of the flux and 10to 90 wt % of the aluminum nitride powder. Such contents of the flux andaluminum nitride powder can reduce a difference in thermal expansionbetween the aluminum nitride sintered bodies and the bonding agent andcan leave a proper amount of the flux at the bonded portion. It istherefore possible to increase the strength and air tightness of thebonded portion. Accordingly, the bonding agent obtained by mixing theflux and aluminum nitride powder contains, preferably, 3 to 69 wt %calcium and 31 to 97 wt % aluminum. More preferably, the bonding agentcontains 40 to 80 wt % of the flux and 20 to 60 wt % of the aluminumnitride powder.

Preferably, maximum particle diameters of the flux and aluminum nitridepowder are 45 μm or less. This can increase the strength and airtightness of the bonded portion. More preferably, the maximum particlediameters of the flux and aluminum nitride powder are 32 μm or less.

[Aluminum Nitride Composite Body]

As shown in FIG. 1, the aluminum nitride composite body 10 according tothe embodiment includes a plurality of aluminum nitride sintered bodies1 and 2 and a bonding layer 3 formed between the plurality of aluminumnitride sintered bodies 1 and 2. The bonding layer 3 contains nitrogen(N), oxygen (O), aluminum (Al), and calcium (Ca), and the content ofrare-earth elements is less than 15 wt %. Such aluminum nitridecomposite body 10 can be manufactured at a low bonding temperature of1500° C. or less, thus resulting in less deformation of the aluminumnitride sintered bodies 1 and 2 and good bonding.

The bonding layer 3 can contain oxides of alkali metal, alkaline earthmetal, a glass forming material, and the like. As the oxides of thegrass forming material, for example, SiO₂, B₂O₃, P₂O₃, or the like canbe contained. When the aluminum nitride composite body is used in thecorrosive gas environment, the content of silica in the bonding layer 3is preferably 10 wt % or less. The content of silica of 10 wt % or lessallows the corrosion resistance to the corrosive gas to be maintained.

Among the oxides contained in the bonding layer 3, the total content ofthe oxides other than the oxide containing at least one of calcium andaluminum is, preferably, less than 20 wt % and, more preferably, 10 wt %or less.

Preferably, the bonding layer 3 contains 15 to 30 wt % nitrogen, 10 to35 wt % oxygen, 20 to 55 wt % aluminum, and 5 to 20 wt % calcium. Such acomposition of the bonding layer 3 can increase the strength and airtightness of the bonded portion. The bonding layer 3 can contain, forexample, a compound N—O—Al—Ca, a compound N—O—Al—Ca—X (X is a rare-earthelement), calcium oxide, aluminum oxide, calcium aluminate, and thelike.

The bonding layer 3 has a thickness of, preferably, 1 to 150 μm and,more preferably, 3 to 100 μm. The maximum particle diameter of aluminumnitride in the bonding layer 3 is, preferably, 45 μm or less and, morepreferably, 32 μm or less. This can increase the strength and airtightness of the bonded portion.

Purities of the aluminum nitride sintered bodies 1 and 2 are,preferably, 85% or more and, more preferably, 90% or more. This canreduce the influence of impurities.

Average particle diameters of the aluminum nitride sintered bodies 1 and2 are, preferably, 0.5 to 15.0 μm and, more preferably, 0.5 to 5.0 μm.Densities of the aluminum nitride sintered bodies 1 and 2 are,preferably, 3.00 to 3.35 g/cm³ and, more preferably, 3.10 to 3.35 g/cm³.This can increase the strength of the aluminum nitride composite body10.

Four point flexural strength (JIS R1601) of the aluminum nitridecomposite body 10 at room temperature is, preferably, 250 MPa or moreand, more preferably, 300 Mpa or more.

The aluminum nitride composite body 10 can be used in a heater 20including a heating element 24 as shown in FIG. 2. The heater 20 isformed of an aluminum composite body including a disk member (plate) 21and a pipe member (shaft) 22 bonded to each other with a bonding layer23 interposed therebetween.

The disk member 21 is formed of an aluminum nitride sintered body. Thedisk member 21 includes a placement surface 21 a where a semiconductorsubstrate (wafer) is placed and a back surface 21 b opposite to theplacement surface 21 a. The disk member 21 includes the heating element24 inside. As the heating element 24, a resistance heating element ofmolybdenum (Mo), tungsten (W), or the like can be used. The heatingelement 24 can be wire-shaped coil-shaped, mesh-shaped, sheet-shaped,and bulk-shaped.

The pipe member 22 is formed of an aluminum nitride sintered body. Thepipe member 22 supports the disk member 21 and accommodates power supplymembers 25 inside a pipe. The pipe member 22 is attached to the backsurface 21 b of the disk member 21 with the bonding layer 23 interposedtherebetween. The power supply members 25 supply electric power to theheating element 24. Ends of the power supply members 25 are connected toterminals of the heating element 24 by brazing or the like.

The bonding layer 23 contains nitrogen, oxygen, aluminum, and calcium,and the content of rare-earth elements is suppressed to less than 15 wt%. The bonding layer 23 bonds the back surface 21 b of the disk member21 and an end surface of the pipe member 22.

Moreover, an electrostatic chuck 30 including an electrode 34 as shownin FIG. 3 can be manufactured using the aluminum nitride composite body10. The electrostatic chuck 30 includes a dielectric layer 31 and a base32 with a bonding layer 33 interposed therebetween.

The dielectric layer 31 and base 32 are made of aluminum nitridesintered bodies. The dielectric layer 31 includes a placement surface 31a where a semiconductor substrate (wafer) is placed and a back surface31 b opposite to the placement surface 31 a. The dielectric layer 31 andbase 32 are bonded to each other with the electrode 34 interposedtherebetween through the bonding layer 33.

The electrode 34 can be made of a high-melting point metal such asmolybdenum (Mo) and tungsten (W). The electrode 34 can be hemispherical,comb-shaped, ring-shaped, mesh-shaped, or bulk-shaped. The electrode 34is connected to a terminal 35 by brazing or the like.

The bonding layer 33 contains nitrogen, oxygen, aluminum, and calcium,and the content of rare-earth elements is suppressed to 15 wt %. Thebonding layer 33 bonds the back surface 31 b of the dielectric layer 31and an upper surface 32 a of the base 32. The bonding layer 33 isprovided so as to fill a gap between the dielectric layer 31 and thebase 32 around the electrode 34.

Using the aluminum nitride composite body, it is possible to manufacturea substance including pipe-shaped aluminum nitride sintered bodiesbonded to each other, a substance including disk-shaped or plate-shapedaluminum nitride sintered bodies bonded to each other, and anelectrostatic chuck which includes a heating element and an electrodeand can perform heating, shapes and applications of which are notlimited.

[Manufacturing Method]

(Bonding Agent)

First, compounds (source of calcium aluminate) for generating at leastcalcium aluminate are measured and mixed. The compounds as the calciumaluminate source can be calcium compounds such as calcium oxide, calciumcarbonate (CaCO₃), and calcium hydroxide (Ca(OH)₂) and aluminumcompounds such as aluminum oxide and aluminum hydroxide (Al(OH)₃). Thesecalcium compounds and aluminum compounds also serve as the calcium oxidesource and the aluminum oxide source, respectively. Preferably, the fluxis prepared by mixing and melting the compounds such that the content ofthe calcium compound is 30 to 70 wt % (calcium oxide equivalent) and thecontent of the aluminum compound is 30 to 70 wt % (aluminum oxideequivalent).

Furthermore, if necessary, the compounds as sources for the alkali metaloxide, alkaline earth metal oxide, and oxide of the glass formingmaterial which can be added may be measured and mixed. The compounds asthe sources for the alkali metal oxide, alkaline earth metal oxide, andoxide of the glass forming material can be hydroxides, oxides, and thelike of alkali metal, alkaline earth metal, and an element contained inthe glass forming material.

The mixed compounds are heated at 1400 to 1600° C. to be melted, thencooled, and crushed to obtain the flux. The thus obtained flux containsat least calcium aluminate. The flux further contains, depending on themixed compounds mixed, heating temperature and time, and the like,calcium oxide, aluminum oxide, alkali metal oxide, alkaline earth metaloxide, the glass forming material, or the like. The flux contains atleast one of Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆ as the calcium aluminate.

Alternatively, calcium oxide and aluminum oxide may be measured andmixed to prepare the flux containing calcium oxide and aluminum oxide.In this case, it is preferable to mix calcium oxide and aluminum oxideat a ratio of 30 to 70 wt % calcium oxide to 30 to 70 wt % aluminumoxide.

In any case, the flux preferably contains 30 to 80 wt % calcium and 20to 70 wt % aluminum. The thus obtained flux and the aluminum nitridepowder are measured and mixed to obtain the bonding agent.

(Aluminum Nitride Sintered Body)

The aluminum nitride powder and, if necessary, a sintering auxiliaryagent are measured, and a binder is added thereto and mixed. Theobtained mixture is molded by a molding method such as CIP (ColdIsostatic Pressing), slip casting, die molding, or the like. In the caseof manufacturing the heater 20 shown in FIG. 2, the heating element 24is embedded in a compact. For example, a preform is formed by using themixture, the heating element 24 is placed on the upper surface of thepreform, and the mixture is filled over the upper surface of the heatingelement 24, thus obtaining the compact with the heating element embeddedtherein.

The obtained compact is baked at 1700 to 2000° C. in an atmosphere ofinert gas such as nitrogen or argon gas or in a reduced pressureatmosphere by a sintering method such as hot pressing and atmosphericsintering. The thus obtained aluminum nitride sintered body isprocessed. The processing is performed such that the average surfaceroughness (Ra) of the bonding surface (a surface in contact with thebonding agent) of the aluminum nitride sintered body is 0.1 to 2.0 μm.This can increase the strength and air tightness of the bonded portion.More preferably, the average surface roughness is 0.4 to 1.2 μm. Thedesired surface roughness can be obtained by processing the aluminumnitride sintered body, for example, by a surface grinding machine, ahigh-speed lapping machine, or the like.

(Aluminum Nitride Composite Body)

The bonding agent containing the flux and aluminum nitride powder isinterposed between the plurality of aluminum nitride sintered bodies andheated at a bonding temperature of 1500° C. or less. This flux containseither calcium aluminate or calcium oxide and aluminum oxide, and thecontent of rare-earth elements is less than 5 wt %.

The boding agent is applied to the bonding surface of each aluminumnitride sintered body. The bonding agent may be applied to one of thealuminum nitride sintered bodies to be bonded or applied to boththereof. The bonding agent can be mixed with IPA (isopropyl alcohol),ethanol, or the like for easy application. Moreover, the bonding agentmay be formed into a sheet and sandwiched between the aluminum nitridesintered bodies. The bonding agent is uniformly applied with a densityof, preferably, 5 to 35 g/cm² and, more preferably, 10 to 30 g/cm².

The bonding surfaces of the aluminum nitride sintered bodies are broughtinto contact with each other with the bonding agent interposedtherebetween and heated at a bonding temperature of 1500° C. or less inan atmosphere of inert gas such as nitrogen gas or argon gas or in areduced-pressure atmosphere. This can provide good bonding at lowbonding temperature and reduce deformation of the aluminum nitridesintered bodies. More preferably, the bonding temperature is 1400 to1500° C. Depending on the size and shape of the aluminum nitridesintered bodies, it is preferable that the aluminum nitride sinteredbodies are held at a bonding temperature of 1500° C. or less for 5minutes to 3 hours.

Preferably, the plurality of aluminum nitride sintered bodies arepressed while the plurality of aluminum nitride sintered bodies are heldat the bonding temperature with the bonding agent interposedtherebetween. This can further increase the strength and air tightnessof the bonded portion. The pressing may be performed during heating tothe bonding temperature and cooling from the bonding temperature. Thepressing is performed in such a manner that the aluminum nitridesintered bodies are pressed to each other in a direction perpendicularto the bonding surfaces. The applied pressure is, depending on the sizeand shape of the aluminum nitride sintered bodies, preferably 5 to 200kg/cm².

Preferably, the aluminum nitride sintered bodies having the bondingagent interposed therebetween are heated up to the bonding temperatureat a heating rate of 0.5 to 10.0° C./min. When the heating rate is lessthan 0.5° C./min, the flux is crystallized, and the melting pointthereof increases, thus degrading the bonding properties of the aluminumnitride sintered bodies. When the heating rate is more than 10.0°C./min, the aluminum nitride composite body would be damaged in themanufacturing process, and the yield thereof is reduced.

As described above, according to the bonding agent of the embodiment andthe method of manufacturing the aluminum nitride composite body usingthe same, it is possible to obtain good bonding at a low bondingtemperature of 1500° C. or less. The obtained aluminum nitride compositebody includes good bonding and less deformation of the aluminum nitridesintered bodies. Moreover, the bonding temperature is low, and it istherefore possible to reduce the energy necessary for bonding andeliminate the need for reprocessing after the bonding, thus dramaticallyreducing manufacturing costs.

When bonding is performed using the bonding agent of the embodiment inthe process of manufacturing the heater including the heating elementand the electrostatic chuck including the electrode, the bonding can becarried out at a low bonding temperature of 1500° C. or less.Accordingly, it is possible to prevent changes in quality of the heatingelement and the electrode and suppress changes of the volume resistivityof the aluminum nitride sintered bodies. It is therefore possible toobtain a heater excellent in temperature uniformity or an electrostaticchuck excellent in uniformity of chucking force.

EXAMPLES

Next, the present invention is described in more detail with examples,but the present invention is not limited to the following examples.

Examples 1 to 11, Comparative Examples 1 to 3

(Aluminum Nitride Sintered Body)

95 wt % of the aluminum nitride powder was added to 5 wt % of yttriumoxide as a sintering auxiliary agent and then mixed using a ball mill.The obtained powder mixture was added to the binder and granulated byspray granulation. The obtained granulated powder was molded into aplate and a pipe by die molding and CIP. The obtained plate compact andpipe compact were baked at 1860° C. for 6 hours in nitrogen gas by hotpressing and in nitrogen gas by atmospheric sintering, respectively.

In terms of size of the obtained aluminum nitride sintered bodies, theplate sintered body had 60 mm length×60 mm width and 20 mm thickness,and the pipe sintered body had 58 mm outer diameter, 20 mm innerdiameter, and 20 mm length. The aluminum nitride sintered bodies wereprocessed such that the flatness was 10 μm or less.

(Bonding Conditions)

Bonding agents shown in Tables 1 and 2 were uniformly applied to thebonding surfaces of the aluminum nitride sintered bodies such that thedensity of the bonding agent was 14 g/cm². The bonding surfaces of thealuminum nitride sintered bodies were brought into contact with eachother with the bonding agent interposed therebetween and held at abonding temperature of 1450° C. in nitrogen gas for 2 hours. The heatingrate was set to 3.3° C./min, and nitrogen gas at 1.5 atm was introducedwhen the temperature reached 1200° C. The aluminum nitride sinteredbodies were pressed to each other in the direction perpendicular to thebonding surface. The pressing was performed at a pressure of 40 kg/cm².The pressing started when the temperature was 1200° C., continued whilethe aluminum nitride sintered bodies were held at the bondingtemperature of 1450° C., and ended when the aluminum nitride sinteredbodies was cooled to 700° C.

(Evaluation Method)

Bonded portions of the obtained aluminum nitride composite bodies wereevaluated in terms of strength, corrosive resistance, air tightness, andpresence of defects. The four-point flexural strength was measured atroom temperature according to JIS R1601. The strength was measuredbefore an endurance test and a corrosion resistance test (hereinafter,referred to as an initial state) and after the endurance test andcorrosion resistance test. The endurance test was performed in such amanner that a process to heat each aluminum nitride composite body to700° C. and cool the same to room temperature was repeated for a hundredtimes in atmosphere.

The corrosion resistance test was performed in such a manner that eachaluminum nitride composite body was held at 540° C. for 5 hours incorrosive gas atmosphere with a pressure of 0.1 Torr. This corrosive gaswas made of NF₃ gas with a flow rate of 75 sccm and nitrogen gas with aflow rate of 100 sccm. In addition, the bonded portions were observedwith a SEM (Scanning Electron Microscope) in the initial state and afterthe corrosion resistance test, and changes were compared. The four-pointflexural strength after the endurance test and the result of the SEMobservation were totally evaluated to make the evaluation of thecorrosion resistance. The Tables 1 and 2 show the four-point flexuralstrength in the initial state and after the endurance test. In theTables 1 and 2, the corrosion resistance was represented by three levelsof “high” indicating high corrosion resistance, “low” indicating lowcorrosion resistance, and “intermediate” indicating intermediatecorrosion resistance therebetween.

In terms of the air tightness, helium gas was introduced from theoutside of each aluminum nitride composite body, and an amount of heliumgas leaking from the bonded portion between the aluminum nitridesintered bodies and flowing into a pipe of the pipe sintered body wasmeasured using a helium leak detector. Moreover, the presence of defectsin the bonding portions was checked using an ultrasonic flaw detector.In the Tables 1 and 2, a ratio of a bonded area to an entire area of aportion to which the bonding agent was applied is shown.

Furthermore, to check the amount of deformation of each aluminum nitridesintered body after the bonding, flatness of the aluminum nitridecomposite body was measured by a three-dimensional measuring machine.The flatness was measured by 17-point measurement before and after thebonding. When the difference in flatness was 30 μm or less, deformationwas judged to be small. The composition of each composite body wasexamined by an EDS (Energy Dispersion Spectroscopy, JED-2200, JEOL,Ltd.). The composition of the flux was analyzed by X-ray diffractionanalysis.

First, fluxes of Examples 1 to 8 were prepared using calcium carbonateand calcium hydroxide as the compounds of the calcium aluminate source,calcium oxide source, and aluminum oxide source for the tests. Thefluxes of Examples 1 to 8 were prepared by measuring and mixing thecompounds at mixing ratios shown in Table 1. In Table 1, the mixingratio of calcium carbonate to calcium hydroxide was shown in calciumoxide equivalent and aluminum oxide equivalent, respectively. ForComparative Examples 1 to 3, these compounds were measured and mixed atmixing ratios shown in Table 1 to prepare fluxes.

The mixed compounds were heated and melted in atmosphere and then putinto water for cooling. The obtained products were then crushed by aball mill so as to pass through a 32 μm sieve, thus preparing thefluxes. Moreover, aluminum nitride powder with a particle diameter of 32μm or less was prepared. Each of the prepared fluxes and the aluminumnitride powder were measured and mixed at a ratio of the flux toaluminum nitride powder of 60 wt %/40 wt % by use of a mortar. Using theobtained bonding agent, the aluminum sintered bodies were bonded to eachother under the above described bonding conditions and then evaluated bythe above evaluation methods. The results thereof are shown in Table 1.The X-ray diffraction diagram of the flux prepared at a mixing ratio ofExample 1 is shown in FIG. 4. In FIG. 4, the vertical axis andhorizontal axis indicate strength (cps) and 2θ (deg.), respectively.TABLE 1 Four-point Flexural Strength (MPa) After Air Flux InitialEndurance Corrosion Tightness (Compound Mixing Ratio) State TestResistance (Torr · l/sec) Defect Example 1 CaO—Al₂O₃ 350 350 High  1 ×10⁻⁸ or less 90% or more (54 wt %/46 wt %) Example 2 CaO—Al₂O₃ 340 340High  1 × 10⁻⁸ or less 90% or more (51 wt %/49 wt %) Example 3 CaO—Al₂O₃260 260 High >1 × 10⁻⁸ Less than 50% (75 wt %/25 wt %) Example 4CaO—Al₂O₃ 240 240 High >1 × 10⁻⁸ Less than 50% (25 wt %/75 wt %) Example5 CaO—Al₂O₃—MgO 300 300 High  1 × 10⁻⁸ or less 90% or more (48 wt %/51wt %/1 wt %) Example 6 CaO—Al₂O₃—SiO₂ 300 160 Intermediate >1 × 10⁻⁸Less than 50% (51 wt %/41 wt %/8 wt %) Example 7 CaO—Al₂O₃—SiO₂ 345 345High >1 × 10⁻⁸ 90% or more (54 wt %/45.7 wt %/0.3 wt %) Example 8CaO—Al₂O₃—SiO₂ 330 300 High >1 × 10⁻⁸ 90% or more (51 wt %/46 wt %/3 wt%) Comparative CaO—Al₂O₃—Y₂O₃ — — — — — Example 1 (43 wt %/52 wt %/5 wt%) Comparative SiO₂—CaO—MgO 160 160 Low >1 × 10⁻⁸ 50% or more Example 2(53 wt %/33 wt %/14 wt %) and less than 90% Comparative SiO₂—MgO—Al₂O₃190 190 Low >1 × 10⁻⁸ 90% or more Example 3 (63 wt %/20 wt %/17 wt %)

As shown in FIG. 4, the flux obtained by mixing and melting calciumcarbonate and calcium hydroxide at the mixing ratio of Example 1contained Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆ as calcium oxide, aluminum oxide, andcalcium aluminate. The same applied to Examples 2 to 4.

As shown in Table 1, Examples 1 to 4 used the fluxes containing calciumoxide and aluminum oxide but not containing any other oxides. In thealuminum nitride composite bodies of Example 1 to 4, the strength atbonded portions was high in the initial state and was not reduced afterthe endurance test.

In terms of the aluminum nitride composite bodies of Examples 1 and 2using the fluxes which were obtained by mixing 40 to 60 wt % calciumcarbonate (calcium oxide equivalent) and 40 to 60 wt % calcium hydroxide(aluminum oxide equivalent) and had a calcium content of 45 to 70 wt %and an aluminum content of 30 to 55 wt %, the four-point flexuralstrength was as high as approx. 300 MPa both in the initial state andafter the endurance test. The composition of the bonding layer ofExample 1 is 25 wt % nitrogen, 13 wt % oxygen, 51 wt % aluminum, 8 wt %calcium, and 3 wt % yttrium. The composition of the bonding layer ofExample 2 is 17 wt % nitrogen, 32 wt % oxygen, 26 wt % aluminum, 17 wt %calcium, and 8 wt % yttrium.

In the aluminum nitride composite bodies of Examples 1 to 4, nocorrosion was observed and the strength was not reduced even after thecorrosion resistance test. The aluminum nitride composite bodies ofExamples 1 to 4 were therefore excellent in corrosion resistance. In thealuminum nitride composite bodies of Examples 1 to 2, the leak amountswere suppressed to 1×10⁻⁸ Torr·1/sec or less to show high air tightnessat the bonded portions. Moreover, in the aluminum nitride compositebodies of Examples 1 to 2, no defects were observed at the bondedportions, and the bonding was very good.

Also the aluminum nitride composite bodies of Examples 5 to 8, whichused bonding agents including the fluxes containing calcium aluminate,calcium oxide, and aluminum oxide and further containing less than 10 wt% of magnesium oxide or silica as the oxide other than the above oxides,had high strength in the initial state. Example 5 using the fluxcontaining magnesium oxide, especially, was not reduced in strengthafter the endurance test and had excellent endurance. Moreover, Example5 had excellent corrosion resistance and air tightness and included nodefects observed at the bonded portion. In terms of the aluminum nitridecomposite bodies of Examples 7 and 8 with silica contents of 0.01 to 5wt %, the four-point flexural strength was as high as about 300 Mpa inthe initial state. Examples 7 and 8 were not reduced in strength afterthe endurance test and had excellent endurance. The corrosion resistancethereof was very high. As described above, it was found that even theflux containing an oxide other than calcium aluminate, calcium oxide,and aluminum oxide can provide good bonding at low bonding temperature.

The aluminum nitride composite bodies of Examples 1 to 8 were alldeformed only slightly by bonding. The SEM observation in the initialstate revealed that the bonding layers had thicknesses of 10 to 80 μmand uniform texture could be obtained.

On the contrary to these Examples 1 to 8, Comparative Example 1, whichused the bonding agent including the flux prepared by mixing calciumoxide, aluminum oxide, and 5 wt % yttrium oxide, could not bond thealuminum nitride sintered bodies to each other at a bonding temperatureof 1450° C.

In Comparative Examples 2 and 3, which used the bonding agents includingthe fluxes prepared by not mixing one of calcium oxide and aluminumoxide, the aluminum nitride sintered bodies could be bonded to eachother but had very low strength at the bonded portions. Furthermore, thealuminum nitride composite bodies of Comparative Examples 2 and 3 hadless corrosion resistance. Moreover, the aluminum nitride compositebodies of Comparative Examples 2 and 3 had leak amounts more than 1×10⁻⁸Torr·1/sec and were less airtight. In Comparative Examples 2 and 3,corrosion of silicon (Si) contained in the fluxes was observed by theSEM.

Furthermore, FIG. 5 shows a SEM photograph of the composite body ofExample 1. As shown in FIG. 5, the bonding agent of Example 1 canprovide the composite body 10 including the aluminum nitride sinteredbodies 1 and 2 firmly bonded to each other with the bonding layer 3interposed therebetween. FIGS. 6 to 10 show results of the EDS analysisof the composition of the composite body of Example 1.

FIGS. 6 to 10 show distributions of nitrogen, oxygen, aluminum, calcium,and yttrium, respectively. In the FIGS. 6 to 10, the number of counts ofsignal intensity of an element targeted for the distribution measurementis shown on the left, and proportions of the signal intensity areindicated on the right. For example, in FIG. 6, the number of counts ofthe signal intensity of the element is 6, 3, 2, 1, and 0 from above, andthe proportions of the signal intensity are 0.45, 1.96, 9.72, 31.61, and56.27.

As shown in FIGS. 6 to 9, the bonding layer contained nitrogen, oxygen,aluminum, and calcium. As shown in FIG. 10, a fraction of yttriumcontained in the aluminum nitride sintered bodies diffused into thebonding layer. As shown in FIG. 9, calcium contained in the bondingagent diffused into the aluminum nitride sintered bodies in the vicinityof the bonding layer.

Next, similar tests were performed using the flux of Example 1 shown inTable 1 with only the mixing ratio of the flux to the aluminum nitridepowder changed as shown in Table 2. Results thereof are shown in Table2. TABLE 2 Four-point Flexural Strength (MPa) Bonding Agent Aftercomposition(wt %) Initial Endurance Corrosion Air Tightness Flux AlNPowder state test Resistance (Torr · l/sec) Defect Example 1 60 40 350350 high  1 × 10⁻⁸ or less 90% or more Example 9 70 30 345 345 high  1 ×10⁻⁸ or less 90% or more Example 10 95 5 240 230 high >1 × 10⁻⁸ 50% ormore and less than 90% Example 11 5 95 230 230 high >1 × 10⁻⁸ less than50%

As shown in Table 2, aluminum nitride composite bodies of Examples 1 and9 to 11 had high strength at bonded portions in the initial state, andthe strength was not reduced after the endurance test. The aluminumnitride composite bodies of Example 1 and 9 to 11 were excellent incorrosion resistance. As described above, good bonding could be obtainedat low bonding temperature with various ratios of the flux to aluminumnitride powder.

The aluminum nitride composite bodies of Examples 1 and 9 containing 10to 90 wt % of the flux and 10 to 90 wt % of the aluminum nitride powder,especially, had four-point flexural strengths of about 350 Mpa both inthe initial state and after the endurance test and were the highest instrength. Moreover, the aluminum nitride composite bodies of Examples 1and 9 achieved best bonding with high air tightness at the bondedportions and no defects observed at the bonded potions.

Example 12

The heater 20 shown in FIG. 2 was manufactured. A compact for the diskmember 21 was formed using the granulated powder obtained in the sameway as that of Examples 1 to 11 by die molding, and a compact for thepipe member 22 was formed by CIP. In the compact for the disk member 21,the heating element 24 which was coil-shaped and made of molybdenum wasembedded. The obtained compact was baked at 1860° C. in nitrogen gas byhot pressing for 6 hours. In terms of size of the obtained aluminumnitride sintered bodies, the disk member 21 had 340 mm diameter and 20mm thickness, and the pipe member 22 had 70 mm outer diameter, 60 mminner diameter, and 180 mm length. The placement surface 21 a of thedisk member 21 was processed to have a flatness of 10 μm or less.

The bonding agent of Example 1 shown in Tables 1 and 2 was uniformlyapplied to the end surface 22 a of the pipe member 22 such that thedensity of the bonding agent was 18 g/cm². The back surface 21 b of thedisk member 21 and the end surface 22 aof the pipe member 22 werebrought into contact with each other with the bonding agent interposedtherebetween and then heated under the conditions similar to those ofExamples 1 to 11 while being pressed. Ends of the heat element 24 werebrazed with ends of the power supply members 25, thus obtaining theheater 20.

The flatness of the placement surface 21 a of the obtained heater 20 wasmeasured by a three dimensional measuring machine. The flatness wasmeasured by 13-point measurement before and after the bonding, and thedifference in flatness was calculated. The difference was 30 μm or less,and the deformation thereof was found to be suppressed to a smallextent. Moreover, measurement of differences in in-plane temperature inthe placement surface 21 a revealed that the variation of thetemperature differences on concentric circles before and after thebonding was suppressed within ±0.5° C. The temperature uniformitythereof was therefore excellent. The measurement of the four-pointflexural strength at room temperature according to JIS R1601 revealedthat the strength was not reduced even by brazing and good bonding wasmaintained.

Although the inventions have been described above by reference tocertain embodiments of the inventions, the inventions are not limited tothe embodiments described above will occur to those skilled in the art,in light of the above teaching.

1. A bonding agent, comprising: a flux containing either calciumaluminate or calcium oxide and aluminum oxide and containing less than 5wt % rare-earth elements; and aluminum nitride powder.
 2. The bondingagent according to claim 1, wherein the calcium aluminate contains atleast one of Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆.
 3. The bonding agent according toclaim 1, wherein the flux contains 30 to 80 wt % calcium and 20 to 70 wt% aluminum.
 4. The bonding agent according to claim 1, wherein the fluxcontains 0.01 to 5 wt % silica.
 5. The bonding agent according to claim1, wherein the content of the flux is 10 to 90 wt % and the content ofthe aluminum nitride powder is 10 to 90 wt %.
 6. The bonding agentaccording to claim 1, wherein maximum particle diameters of the flux andthe aluminum nitride powder are 45 μm or less.
 7. An aluminum nitridecomposite body, comprising: a plurality of aluminum nitride sinteredbodies; and a bonding layer formed between each adjacent pair of theplurality of aluminum nitride sintered bodies, the bonding layercontaining nitrogen, oxygen, aluminum and calcium and containing lessthan 15 wt % rare-earth elements, wherein the plurality of aluminumnitride sintered bodies are bonded through the bonding layer.
 8. Thealuminum nitride composite body according to claim 7, wherein thebonding layer contains 15 to 30 wt % nitrogen, 10 to 35 wt % oxygen, 20to 55 wt % aluminum, and 5 to 20 wt % calcium.
 9. The aluminum nitridecomposite body according to claim 7, wherein the aluminum nitridecomposite body is used for a heater including a heating element.
 10. Thealuminum nitride composite body according to claim 7, wherein thealuminum nitride composite body is used for an electrostatic chuckincluding an electrode.
 11. A method of manufacturing an aluminumnitride composite body, comprising: heating a plurality of aluminumnitride sintered bodies with a bonding agent interposed therebetween ata bonding temperature of 1500° C. or less to melt the bonding agent; andbonding the plurality of aluminum nitride sintered bodies to each other,wherein the bonding agent contains a flux and aluminum nitride powder,and the flux contains either calcium aluminate or calcium oxide andaluminum oxide and contains less than 5 wt % rare-earth elements. 12.The method of manufacturing an aluminum nitride composite body accordingto claim 11, wherein the plurality of aluminum nitride sintered bodieswith the bonding agent interposed therebeween are heated to the bondingtemperature at a heating rate of 0.5 to 10.0C./min.
 13. The method ofmanufacturing an aluminum nitride composite body according to claim 11,wherein an average surface roughness of a bonding surface of each of thealuminum nitride sintered bodies is 0.1 to 2.0 μm.